This invention relates to a medical electrode. It relates more particularly to a disposable premoistened electrode for adherence to a patient's body to provide conductive contact between an area of the patient's skin underlying the electrode and apparatus for monitoring electrical signal originating in the patient's body.
To maximize the strength of the body signals applied to the monitoring apparatus, it is essential that a good conductive path be provided between the electrical lead coupled to the electrode and leading to the monitoring apparatus and the patient's skin area underlying the electrode.
Conventional electrodes of this type typically have a support layer with an adhesive underside which supports a conductive contact element in the form of a conductive male snap fastener element. The bottom face of the snap fastener element is exposed at the underside of the support layer while its tip projects from the top surface of the support layer so that the tip can be coupled to a female snap fastener element connected by an electrical lead to the monitoring equipment.
The support layer also supports a resilient pad impregnated with a conductive gel. The upper surface of the pad makes good conductive contact with the underside of the snap fastener element and when the support layer is adhered to the patient's body, the underside of the gel pad makes good conductive contact with the skin area against which it is pressed.
One problem encountered with the prior medical electrodes of this general type is corrosion of the part of the snap fastener element contacted by the gel pad. This corrosion results because the electrolyte gel in the pad often contains salts which corrode the metal in the fastener element. The corrosion, in turn, gives rise to noise artifacts in the signal coupled by the electrode to the monitoring equipment and increases the impedance of the electrode.
To overcome this problem, prior electrodes employ a snap fastener element whose eyelet is made of noncorroding silver or which is made entirely of silver. This has proved to be a relatively expensive solution to the corrosion problem because of the high cost of silver today.
In an attempt to achieve the same results at lower cost, one prior electrode we are aware of has a snap fastener element made entirely of plastic which is then silver coated. In other words, the fastener rivet stud and eyelet are separately molded plastic pieces which are then coated with silver. When the plastic stud and eyelet are press-fit together, the silver coating thereon does provide a reasonably good conductive path between the gel pad and the mating snap fastener element leading to the monitoring equipment.
However, in a given monitoring application, it may be necessary to repeatedly connect the monitoring equipment to the snap fastener element on the electrode because the patient is being moved or because it is necessary to connect a particular electrode to different pieces of equipment. In fact, when meeting specifications, it is often necessary to guarantee that the electrode will be able to withstand a specified number of connect and disconnect operations, e.g. six, without any material change in its impedance or d.c. offset. The electrodes employing the aforesaid silver-plated plastic fastener elements sometimes fail to meet this test. This is because the repeated connections to the fastener stud abrades the silver coating thereon. Moreover, those prior plated fastener elements are still relatively expensive because of the amount of silver on them.
Another factor which adversely affects the impedance characteristics of prior electrodes involves loss of electrolyte gel which sometimes occurs when the electrode is adhered to the patient's body. More particularly, when adhering the electrode, or connecting its contact element, the resilient gel pad is compressed against the skin, with the result that some of the gel is squeezed out of the pad and along the skin. Then when the pad resumes its unstressed condition, there may be some loss of gel at the boundary between the gel pad and the snap fastener element so that there is no longer good conductive contact between the element and pad.
Attempts to overcome this problem have involved forming a rigid wall around the gel pad. When the electrode is adhered to the patient's body, the wall is pressed against the skin and acts as a barrier to prevent the gel from being squeezed out of the pad. An example of this type of electrode is described in U.S. Pat. No. 3,838,766.
While this rigid wall does contain the electrolyte gel, it also causes patient discomfort because it makes the electrode as a whole relatively rigid and nonconforming. It also bears against the skin and becomes irritating, particularly to an infant or a patient with particularly tender skin. Such electrodes have other disadvantages fully described in the aforesaid patent. Furthermore, they have a relatively high profile, making them rather bulky and prone to catch on objects that might strip the electrode from the patient's body.